1. Field of the Invention
The present invention relates to an improvement in a dome connecting structure of an external-combustion hot stove which is annexed to a blast furnace.
More particularly, the present invention is concerned with a structure for mounting a dome connecting member which interconnects separate domes constituting a combustion chamber and a heat accumulation chamber and which does not incorporate any expansion joint.
2. Description of the Related Art
In general, hot stoves for stably supplying hot air at high temperature to a blast furnace have a combustion chamber and a heat accumulation chamber. Hot stoves can broadly be sorted into two types: (a) internal combustion type hot stoves in which the combustion chamber and the heat accumulation chamber are consolidated and (b) external combustion type hot stoves in which these chambers are arranged separately from each other.
The external combustion type is becoming dominant in pace with the current trend for greater scales or sizes of blast furnaces.
Hot stoves of the external combustion type employ a combustion chamber into which blast furnace gas or coke oven gas is introduced to be burnt therein, and also employ a heat accumulation chamber in which the heat generated by the combustion is accumulated.. These chambers are interconnected through a furnace top dome so as to enable accumulation of heat and supply of hot gas the blast furnace.
A conventional connecting structure known as the Koppers type structure employs a straight connecting pipe which interconnects the side walls of a heat accumulation chamber dome and a combustion chamber dome and which incorporates an expansion joint. This type of connecting structure is disclosed, for example, in Japanese Patent Laid-Open Nos. 50-104707 and 53-131906, as well as in Japanese Utility Model Laid-Open No. 55-4285.
FIG. 9 of the drawings shows an example of a hot stove of the Koppers type. This hot stove, denoted by the number 12, has a heat accumulation chamber 1 which is of the self-standing type, and has a combustion chamber 2 situated on a column 3. A dome 9 on the heat accumulation chamber 1 and a dome 10 on the combustion chamber 2 are connected to each other through a dome connecting pipe 4 having an expansion joint 5. When supplying hot gas, an axial force of about 1000 tons acts due to pressure of the gas, tending to move domes 9, 10 away from each other. In order to prevent excessive expansion of the expansion joint 5, therefore, both the heat accumulation chamber dome 9 and the combustion chamber dome 10 are provided with dome ring stiffeners 6 which are interconnected to each other through a tension beam 7. Numeral 8 designates a connection truss which interconnects the heat accumulation chamber 1 and the combustion chamber 2.
As stated above, the hot stove thus constructed produces an axial force of about 1000 tons which acts to tension the tensionbeam 7 during supply of the hot gas. Consequently, the connecting structure, in particular the expansion joint 5, experiences repeated alternating cycles of expansion caused by the pressure of the gas during gas supply and contraction due to gas pressure reduction during combustion. This continues for a very long period, which is usually about 10 to 15 years.
Consequently, refractory bricks lining the expansion joint 5 are caused to crack and to form cavities through which the hot gas reaches the outer shell of the expansion joint 5 making the shell red hot.
Repairing such a damaged expansion joint 5 requires the operator to suspend the operation of the hot stove for a long time so as to cool down the hot stove to enable renewal of the expansion joint 5, during which the number of hot stoves available for operation is reduced, e.g., from 4 to 3, with the result that the efficient operation of the blast furnace is uneconomically affected and much time and huge cost are required.
Simplified and light-weight construction of hot stoves also has been demanded to enable periodic draining of water from the lowest portion of the expansion joint, and to facilitate maintenance.
In order to cope with such a demand while obviating the above-described problems, various connecting structures have been proposed in which thermal expansion is absorbed solely by the rigidity of a connecting pipe, without the aid of the expansion joint. An example of a known hot stove incorporating a dome connecting pipe 4 devoid of expansion joint 5 is shown in FIG. 10. It will be seen that the dome 9 of the heat accumulation chamber 1 and the dome 10 of the combustion chamber 2 are directly connected to each other through a dome connecting pipe 4 which does not have the expansion joint 5 used in the conventional structure shown in FIG. 9. In addition, the dome ring stiffeners 6 and the tension beam 7 used in conventional structures are omitted. Thus, the whole construction is simplified and light-weight due to elimination of the dome ring stiffeners 6 and the tension beam 7, not to mention omission of the expansion joint 5.
In the structure shown in FIG. 10, the elongation of the iron shell of the dome connecting pipe 4 is absorbed by distortions of the heat accumulation chamber 1 and the combustion chamber 2, so that no problem is caused in regard to thermal stress. A problem, however, arises when the hot stove is of the type in which the heat accumulation chamber 1 is of self-standing type while the combustion chamber 2 is situated on the column 3. In such a hot stove stacking of the refractory bricks in the combustion chamber 2 is conducted after the dome 9 of the heat accumulation chamber 1 and the dome 10 of the combustion chamber 2 are connected to each other through the dome connecting pipe 4, so that the column 3 is deflected by the weight of the refractory bricks, with the result that the level of the combustion chamber 2 is lowered with respect to that of the heat accumulation chamber 1.
FIGS. 11A and 11B illustrate the results of an FEM (finite-element method) analysis of stress and deformation at the portion of the heat accumulation chamber dome 9 near the base A of the dome connecting pipe 4 and those at the portion of the combustion chamber dome 10 near the base B of the dome connecting pipe 4. From these results it is understood that the portion of the combustion chamber dome 10 near the base B of the dome connecting pipe 4 suffers from large stress and deformation, whereas the stress and deformation are rather small in the portion of the heat accumulation chamber dome 9 near the base A of the dome connecting pipe 4. Consequently, a substantial reinforcement is required at the knuckle part 11 (FIG. 10) of the portion of the combustion chamber 2 near the base end of the dome connecting pipe 4. In the worst case, it is impossible to build the connecting structure without the expansion joint, thus failing to achieve the intended purpose.